In the grinding process, the surface layer of the metal that changes in the physical, chemical and mechanical properties caused by grinding force and heat is called the metamorphic layer. Grinding forces and grinding heat are the key to causing grinding burns, creating grinding cracks and determining the depth of the metamorphic layer. Shanghai Huanyue Machine Tool shares an article with you, for the purpose of reducing the tooth surface defects of a hard tooth surface gear of a machine, systematically researching the mechanism of tooth surface defects, analyzing the factors affecting tooth surface defects, adjusting the process flow, optimizing process parameters, and proposing control and improvement measures to reduce the grinding defects of hard gear tooth surface.
First of all, let's understand the mechanism of tooth surface grinding cracks. When grinding the tooth flank, the grinding temperature of the tooth flank of the part can reach 820 840. The material structure of the part is martensite and residual austenite. The residual austenite decomposes under the influence of grinding heat during grinding, and gradually transforms into martensite and collects on the surface, causing local expansion and brittleness of the part, which increases the surface stress of the part and leads to the formation of grinding cracks.
The following are the defect acceptance requirements after grinding the tooth surface of a certain machine:
1.The defect-free surface has a uniform, matte medium-gray appearance.
2.All decarburized parts of the gear shine after nitric acid corrosion.
1) White patches. Spots with very clear boundaries. This spot is not caused by contamination in carburizing, but by contamination during heat treatment.
2) Carburizing soft spots. These defects appear as light gray or white areas, usually round, and are caused by local lack of carbon penetration due to contamination during carburizing.
3) Micro-decarburization. Such defects appear as bright areas, mixed with normal gray corroded surfaces, light gray areas without specific properties, and have no clear boundaries. This is generally caused by deformation of the part or an error in the clamping.
4) Grinding defects. This defect manifests itself as grinding softening and grinding re-hardening, which is caused by local overheating generated during the grinding process.
Grinding and softening. This defect appears as a darker gray or black area or spot compared to a normal gray corroded surface.
Grinding and re-hardening. Such defects appear as very light gray or white stripes or spots with clear demarcation lines, and are usually surrounded by softened areas.
Abrasive scratches. This defect is manifested as a very light streak, which is generally much longer than the rehardening of the abrasive, and is caused by the gnawing and scratching of fine sand particles falling between the grinding wheel and the workpiece.
5) Grinding cracks.
A single display. Adjacent displays are considered to be single displays when their circumferential spacing is greater than the length of the larger display, and their axial spacing is greater than 3 times the length of the larger display.
Discontinuous display. The axial spacing of the display distributed in the main chain on a line is less than the axial spacing specified for the individual display.
At present, magnetic particle inspection is the most commonly used method for checking grinding cracks.
What are the main process measures that can be taken to reduce the burns and cracks of tooth surface grinding?
1.Reduce the carbon concentration on the surface of the part.
In the carburizing process, if the atmosphere control of carbon is weak, the carbon concentration on the surface of the part will be greatly increased. The high carbon concentration will cause the metallographic structure of the part to manifest carbides, resulting in the precipitation of residual austenite. The surface of the part will be extruded during grinding, and the residual austenite structure will be converted into martensite, and the surface of the part will be subjected to high grinding temperature to produce tensile stress, resulting in grinding defects.
2.Increase the cooling rate of the part after carburizing.
Usually, it is cooled to room temperature in air after carburizing. In this case, the cooling rate must be controlled, and too slow will cause the carburized layer to form a network and block carbide, which can easily cause grinding defects.
3.Appropriately increase the stress relief process.
After carburizing and quenching on the surface of the part, the metallographic austenite will be converted into martensite in an expanded state, but the grinding heat will accelerate the tempering shrinkage of martensite, which will form tensile stress on the surface of the part, resulting in the formation of grinding defects on the surface of the part. Only by relieving the tempering can the tempering time be extended and the size of the martensite can be controlled, so as to reduce the content of residual austenite, reduce the residual stress, and reduce the occurrence of grinding defects.
4.Choose a reasonable carburizing and quenching cooling medium.
In the carburizing and quenching process of parts, it is critical to control the martensitic structure of the carburized layer. Only a reasonable selection of cooling medium can control the stress generated during the martensitic transition and reduce the heat treatment deformation.
5.Reasonable choice of grinding wheels.
1) On the premise that the particle size of the grinding wheel can meet the roughness requirements of the parts, the coarse-grained grinding wheel should be selected as much as possible;
2) Choose soft grinding wheel for parts with high hardness, and choose soft grinding wheel for parts with low hardness;
3) Choose a grinding wheel with a smaller diameter as much as possible in production;
4) The use of slotted grinding wheel for intermittent grinding can increase heat dissipation and fully remove chips;
5) Increase the frequency of dressing the grinding wheel to improve the balance accuracy of the grinding wheel.
6.Control of grinding parameters.
If burns occur when the grinding amount is small, the longitudinal feed rate can be increased. If burns occur when the grinding amount is large, the feed should be reduced, the grinding times should be increased, and the grinding feed should be gradually reduced, so as to eliminate the defect layer generated in the early grinding and reduce the occurrence of grinding defects.
7.Reasonable selection of cooling method and coolant for grinding process.
Selection of cooling method: The grinding wheel is effectively cooled and cleaned to remove the heat generated by grinding, prevent the grinding wheel from clogging, maintain the cleanliness of the grinding fluid, and reduce the occurrence of grinding defects.
Coolant selection: Coolant must have good cooling, lubrication and cleaning properties. Good cooling means that it can remove heat from the machining area, reduce the temperature of the machining area, and prevent grinding defects in the part. Good lubricity refers to the formation of a lubricating film between the grinding surface and the abrasive grain, reducing the direct friction between the part and the grinding wheel. Good cleaning performance means that the grinding debris generated by processing can be washed out in time to prevent or alleviate the clogging of the grinding wheel.
8.Control the dressing of the grinding wheel.
The coarser the grinding wheel is trimmed, the more cracks form on the grinding wheel section. During the grinding process, the better the heat dissipation, the less likely the grinding wheel is to clog, which has a positive effect on avoiding burns and cracks on the surface of the parts.
The study of gear flank defects is a huge field involving many aspects of knowledge, and there are many technical issues that deserve further in-depth study. First, gear material improvement. In the later development, composite materials or engineering plastics with light weight, vibration absorption and self-lubricating properties will be gradually used to replace carbon steel materials. Second, the research level of abrasives and abrasives has been continuously improved. With the advent of a new generation of composite abrasives, the renewal of coolant, the optimization level of tooth surface defects and the processing efficiency will be greatly improved.
*: Geared.